Dithiophosphorus acids are useful extractants for the solvent extraction of metal cations. The dithiophosphorus acids of the specification are represented by the following: ##STR1## wherein R.sup.1 and R.sup.2 are the same or different and are selected from the group consisting of substituted alkyl, cycloalkyl, alkoxyalkyl, alkylcycloalkyl, aryl, alkylaryl, aralkyl and cycloalkylaryl radicals having from about 2 to 24 carbon atoms.
Unfortunately, the dithiophosphorus acid extractants are prone to oxidation during the solvent extraction process. When oxidized, two acid extractant molecules form a sulfur--sulfur bond leading to a disulfide which is incapable of extracting metal cations. These dithiophosphorus acid extractants would not be economic reagents, unless some method of regenerating the active extractant from the disulfide is available.
Rickelton et al., in U.S. Pat. No. 5,759,512 ('512), disclose a process that relies upon active nascent hydrogen to break the sulfur--sulfur bond of the disulfide and regenerate the acid extractant. For example, this process adds zinc powder to an agitated mixture of sulfuric acid solution and the solvent extractant organic solution, generating nascent hydrogen for the extractant regeneration such that all the metal is finally dissolved. Although relatively costly, this regeneration improves the economics of recovering metals with dithiophosphorus extractants, since the partially oxidized extractant solution can be regenerated and, therefore, the original metal loading capacity can be restored.
Specifically, the process of the '512 patent relies upon either nascent hydrogen formed from reacting a metal with a strong acid in aqueous phase or from bubbled hydrogen in the presence of a catalyst. Unfortunately, these processes generate large quantities of unreacted hydrogen gas that require special equipment and precautions to prevent explosions. Furthermore, the nascent hydrogen processes, such as the zinc-sulfuric acid process, require neutralization and disposal of an acidic waste product. Moreover, the metal consumption described in the '512 patent is much larger than what appears to be required by the extractant regeneration stoichiometry, suggesting a poor efficiency of the nascent hydrogen towards regeneration versus hydrogen gas formation.
Denger et al., in "Synthesis, Properties and Structure of Bis (dialkyldithiophosphinato) manganese (II) Complexes," Inorganica Chemica Acta, 132 (1987), disclose reacting powdered manganese with bis(diorganothiophosphoryl) disulf[ide] to form laboratory scale quantities of a manganese (II) dithiophosphinate for x-ray crystallographic studies.
It is an object of the invention to provide a process that regenerates disulfides formed by oxidation of dithiophosphorus acids without reliance upon nascent hydrogen or a catalyst.
It is a further object of the invention to provide a method of regenerating the disulfides formed by oxidation of dithiophosphorus acids without the necessity of having to dispose of an acidic waste stream.
It is a further object of the invention to reduce the amount of metal consumed during regeneration, thereby resulting in a lower cost process.